Two quick-fire questions on spotting chemical reactions.
(a)Which of these is most likely to show that a chemical reaction has taken place?(1)
A) the substance changes shape
B) a gas is given off and bubbles are seen
C) the substance dissolves in water
D) the substance is cut into smaller pieces
(b)A blue solid is heated and turns into a white solid, giving off a gas. Which observation is best evidence of a chemical change rather than a physical one?(1)
A) the colour change and gas given off
B) the solid getting hot
C) the container getting lighter
D) the solid staying the same shape
2
Complete each word equation by filling in the missing word or words.
State whether each sentence about chemical reactions is true or false.
(a)In a chemical reaction, at least one new substance is always formed.(1)
(b)The total mass of the reactants is always greater than the total mass of the products.(1)
(c)Every chemical reaction can be reversed simply by returning the products to room temperature.(1)
4
Balance each symbol equation by writing the correct numbers in front of the formulae.
(a)___Mg + ___O2 -> ___MgO(2)
(b)___Na + ___Cl2 -> ___NaCl(2)
5
10.0 g of solid calcium carbonate is heated strongly in an open test tube until it has fully decomposed into calcium oxide and carbon dioxide gas, which escapes. The solid remaining (calcium oxide) has a mass of 5.6 g.
(a)Calculate the mass of carbon dioxide gas released during heating.(2)
(b)Explain what would happen to the total mass measured on a balance if this same reaction were carried out in a sealed, unbreakable container instead of an open test tube.(2)
6
Required practical: Priya heats a coiled strip of magnesium ribbon in a crucible with a lid, lifting the lid briefly from time to time to let air in, then weighs the crucible before and after heating. The empty crucible has a mass of 25.00 g. The crucible plus magnesium ribbon has a mass of 25.40 g before heating. After the magnesium has burned completely, the crucible plus the white solid formed has a mass of 25.65 g.
(a)Name a piece of equipment, other than the crucible, used to lift the hot lid safely during this practical.(1)
(b)Explain why the mass of the solid in the crucible increases after the magnesium ribbon has burned completely, even though no new atoms are created.(2)
(c)Calculate the mass of magnesium used, the mass of magnesium oxide formed, and the mass of oxygen gained by the magnesium.(3)
(d)In one group's experiment, the lid was left off completely and some white smoke was seen escaping from the crucible during heating. Evaluate how this would affect their calculated mass of oxygen gained, and suggest one improvement to the method.(2)
7
Classify each process below as exothermic or endothermic.
(a)Methane gas burning in a Bunsen burner flame.(1)
(b)Photosynthesis taking place in a green leaf.(1)
(c)Calcium carbonate thermally decomposing on strong heating.(1)
(d)Iron powder reacting slowly with oxygen inside a hand warmer sachet.(1)
8
Required practical: Tom investigates temperature changes in reactions. He adds 25 cm3 of dilute hydrochloric acid to 25 cm3 of sodium hydroxide solution in a polystyrene cup, and separately dissolves ammonium nitrate solid in water in a second cup, recording the temperature before and after each.
(a)Name the piece of apparatus used to measure the temperature change in this practical.(1)
(b)In the acid and alkali experiment, the temperature was 20.0C before mixing and 27.5C at its highest after mixing. Calculate the temperature change.(1)
(c)State and explain whether this neutralisation reaction is exothermic or endothermic.(2)
(d)In the second experiment, dissolving ammonium nitrate, the temperature fell from 20.0C to 14.0C. Calculate the temperature change and identify the type of reaction.(2)
9
Required practical: Sam reacts marble chips (calcium carbonate) with excess dilute hydrochloric acid in a conical flask connected to a gas syringe, which measures the volume of carbon dioxide gas produced. Results: Time (s): 0, 20, 40, 60, 80, 100, 120. Volume of gas (cm3): 0, 20, 34, 48, 54, 58, 60.
(a)Identify the independent variable and the dependent variable in this investigation.(2)
(b)Calculate the mean rate of reaction between 0 and 60 seconds, in cm3/s.(2)
(c)Explain why the rate of reaction decreases as the reaction proceeds, and why all three test tubes eventually stop producing gas.(2)
10
State what each pH value shows when tested with universal indicator.
(a)pH 1(1)
(b)pH 6(1)
(c)pH 7(1)
(d)pH 13(1)
11
Complete each neutralisation word equation, including the name of the salt formed.
Required practical: a technician prepares pure, dry crystals of copper sulfate by reacting excess copper oxide powder with warm dilute sulfuric acid, then filters, evaporates, cools and dries the crystals formed.
(a)Put these steps of the method in the correct order by writing 1 to 5 next to each: (i) leave the solution to cool so crystals form, then filter and wash them (ii) filter the mixture to remove the excess unreacted copper oxide (iii) add excess copper oxide powder to warm dilute sulfuric acid and stir (iv) dry the crystals between sheets of filter paper (v) gently heat the filtered solution to evaporate water until crystallisation point is reached(1)
(b)Explain why excess copper oxide, rather than an exact measured amount, is added to the sulfuric acid.(2)
(c)A student's final crystals were still slightly damp and appeared pale blue-green rather than bright blue, suggesting some unreacted copper oxide remained mixed in. Suggest one change to the method that would improve the purity of the final crystals.(2)
13
2.0 cm3 of a dilute acid exactly neutralises 5.0 cm3 of an alkali. Assume the same ratio applies whenever these two solutions are mixed.
(a)Calculate the volume of acid needed to exactly neutralise 15.0 cm3 of the same alkali.(2)
(b)A student accidentally adds 8.0 cm3 of acid to the 15.0 cm3 of alkali, past the exact neutral point. State what would happen to the pH of the mixture and the colour shown by universal indicator, compared with the exact neutral point.(2)
14
The reactivity series can be used to predict how vigorously different metals react.
(a)Using the reactivity series you have learned, list these four metals in order of reactivity, most reactive first: copper, magnesium, zinc, iron.(2)
(b)A student reacts small, equal-sized samples of four unknown metals W, X, Y and Z with dilute hydrochloric acid and records their observations: W bubbles vigorously straightaway; X bubbles steadily; Y produces only a few slow bubbles; Z shows no reaction at all. Deduce the order of reactivity of these four metals, most reactive first.(2)
15
An iron nail is placed into a test tube of blue copper sulfate solution.
(a)Complete the word equation for the reaction that takes place: iron + copper sulfate -> ____________ + ____________(2)
(b)Explain, using the reactivity series, why this displacement reaction happens.(2)
(c)Predict and explain what would happen (if anything) if a strip of copper metal were placed into a solution of iron sulfate instead.(2)
16
Required practical: three sealed test tubes are set up to investigate the conditions needed for rusting. Tube A contains an iron nail with a drying agent, in dry air. Tube B contains an iron nail fully covered by boiled (cooled) water with a layer of oil on top. Tube C contains an iron nail in water that is open to the air. All three tubes are left for one week.
(a)State the two conditions that must both be present for iron to rust.(2)
(b)Identify which tube(s) would show rusting after one week, and explain why the other tube(s) do not rust.(3)
(c)Suggest one variable that should be controlled to make this a fair test.(2)
17
Iron and steel structures can be protected from rusting in several ways: painting the surface, galvanising with a layer of zinc, and sacrificial protection using blocks of a more reactive metal such as magnesium. Compare and evaluate these three methods, and use your knowledge of reactivity and rusting to suggest which would be most suitable for protecting the steel hull of a large ship that spends most of its time in seawater.
(6)
18
Hydrogen peroxide solution decomposes very slowly at room temperature, but decomposes rapidly when a small amount of manganese oxide powder is added.
(a)Define the term catalyst.(1)
(b)Using collision theory, explain how the manganese oxide catalyst increases the rate of this reaction.(2)
(c)A catalyst lowers the activation energy needed for a reaction to happen. Explain what this means for the proportion of particle collisions that result in a reaction, compared with the uncatalysed reaction.(2)
19
Methane gas burns completely in a plentiful supply of oxygen.
(a)Complete the word equation for the complete combustion of methane: methane + oxygen -> ____________ + ____________(1)
(b)Balance the symbol equation for this reaction: ___CH4 + ___O2 -> ___CO2 + ___H2O(2)
(c)4.00 g of methane reacts completely with 16.00 g of oxygen gas, producing 11.00 g of carbon dioxide and some water. Calculate the mass of water produced.(2)
20
A 2.40 g strip of magnesium ribbon is burned completely in air and forms 4.00 g of magnesium oxide.
(a)Show that the percentage increase in mass of the magnesium is 66.7% (to 3 significant figures).(2)
(b)The relative formula mass of magnesium oxide (MgO) is 40, and the relative atomic mass of magnesium is 24. Calculate the relative atomic mass of oxygen in this compound, showing your reasoning.(2)
21
A graph shows the volume of gas produced over time when a small, fixed mass of marble chips (the limiting reactant) reacts with an excess of hydrochloric acid at three different concentrations: dilute (curve A), medium (curve B) and concentrated (curve C). All other variables are kept the same. Curve C rises most steeply and levels off soonest; curve A rises least steeply and takes longest to level off; all three curves eventually level off at the same final volume of gas.
(6)
Mark scheme · K8 Chemical Reactions
Question 1
(a) B1 B cao
(a) Answer: B
(b) B1 A cao
(b) Answer: A
Question 2
(a) B1 oxygen cao
(a) Answer: oxygen
(b) B1 iron sulfide, oe cao
(b) Answer: iron sulfide
(c) B1 methane (or another named hydrocarbon fuel), oe cao
(c) Answer: methane
Question 3
(a) B1 true cao
(a) Answer: True
(b) B1 false cao
(b) Answer: False
(c) B1 false cao
(c) Answer: False
Question 4
(a) B1 2 in front of Mg on the left
(a) B1 2 in front of MgO on the right, fully balanced: 2Mg + O2 -> 2MgO, oe cao
(a) Answer: 2Mg + O2 -> 2MgO
(b) B1 2 in front of Na on the left
(b) B1 2 in front of NaCl on the right, fully balanced: 2Na + Cl2 -> 2NaCl, oe cao
(b) Answer: 2Na + Cl2 -> 2NaCl
Question 5
(a) M1 10.0 - 5.6
(a) A1 4.4 (g) cao
(a) Answer: 4.4 g
(b) B1 the total mass would stay the same (would not decrease), oe
(b) B1 because the carbon dioxide gas cannot escape and is still weighed as part of the sealed system, so no atoms are lost, oe
(b) Answer: The total mass would stay the same, because the carbon dioxide gas produced could not escape and would still be weighed inside the sealed container.
Question 6
(a) B1 tongs cao
(a) Answer: Tongs
(b) B1 the magnesium reacts with (joins with) oxygen from the air to form magnesium oxide, oe
(b) B1 the mass of oxygen added becomes part of the solid product, so the solid's mass increases, oe
(b) Answer: Magnesium combines with oxygen from the air to form magnesium oxide; the mass of oxygen that joins the magnesium adds to the mass of the solid left in the crucible.
(c) M1 mass Mg = 25.40 - 25.00 = 0.40 (g)
(c) M1 mass MgO = 25.65 - 25.00 = 0.65 (g)
(c) A1 mass oxygen gained = 0.65 - 0.40 = 0.25 (g) cao
(d) B1 the calculated mass of oxygen gained (and mass of magnesium oxide) would appear lower than the true value, because some magnesium oxide particles (smoke) escaped before the final mass was measured, oe
(d) B1 improvement: keep the lid on the crucible, only lifting it briefly from time to time to let more air in, oe
(d) Answer: The mass of oxygen gained would be underestimated, since some magnesium oxide smoke escaped and was not weighed. Improvement: keep the lid mostly on, lifting it only briefly to let air in.
Question 7
(a) B1 exothermic cao
(a) Answer: Exothermic
(b) B1 endothermic cao
(b) Answer: Endothermic
(c) B1 endothermic cao
(c) Answer: Endothermic
(d) B1 exothermic cao
(d) Answer: Exothermic
Question 8
(a) B1 thermometer cao
(a) Answer: Thermometer
(b) B1 7.5 (C) cao
(b) Answer: 7.5 C increase
(c) B1 exothermic cao
(c) B1 the temperature increased, showing energy was transferred from the reacting chemicals to the surroundings (the solution), oe
(c) Answer: Exothermic; the temperature rose by 7.5C, showing that energy was released to the surroundings during neutralisation.
(d) M1 20.0 - 14.0
(d) A1 6.0 (C) decrease; endothermic cao
(d) Answer: 6.0 C decrease; endothermic
Question 9
(a) B1 independent variable: time cao
(a) B1 dependent variable: volume of gas (produced) cao
(a) Answer: Independent variable: time. Dependent variable: volume of gas produced.
(b) M1 48 / 60
(b) A1 0.8 (cm3/s) cao
(b) Answer: 0.8 cm3/s
(c) B1 the concentration of the acid decreases as it is used up, so there are fewer acid particles per unit volume, oe
(c) B1 this means collisions between acid particles and the marble chip surface happen less often, so fewer successful collisions occur per second, and the reaction eventually stops once all the acid or marble has been used up, oe
(c) Answer: As the acid is used up its concentration falls, so acid particles collide with the marble chip surface less often, reducing the rate; the reaction stops once one of the reactants is fully used up.
Question 10
(a) B1 strong acid cao
(a) Answer: Strong acid
(b) B1 weak acid cao
(b) Answer: Weak acid
(c) B1 neutral cao
(c) Answer: Neutral
(d) B1 strong alkali cao
(d) Answer: Strong alkali
Question 11
(a) B1 sodium chloride, oe
(a) B1 correctly links hydrochloric acid to a chloride salt, cao
(a) Answer: sodium chloride
(b) B1 potassium sulfate, oe
(b) B1 correctly links sulfuric acid to a sulfate salt (not sulfide or sulfite), cao
(b) Answer: potassium sulfate
Question 12
(a) B1 iii, ii, v, i, iv (fully correct order only) cao
(a) Answer: (iii), (ii), (v), (i), (iv)
(b) B1 using excess copper oxide ensures that all of the sulfuric acid is used up (fully reacted/neutralised), oe
(b) B1 any leftover solid copper oxide is easily removed by filtration, whereas leftover acid in the solution could not easily be removed, oe
(b) Answer: Excess copper oxide guarantees that all of the acid reacts completely; the unreacted solid left over is easily filtered out, but leftover acid would be much harder to remove from the crystals.
(c) B1 filter the mixture more thoroughly or repeat the filtration to remove all the remaining solid copper oxide before evaporating, oe
(c) B1 dry the crystals for longer, or pat them dry between fresh sheets of filter paper rather than leaving them damp, oe
(c) Answer: Filter the solution more carefully (or refilter it) to remove all remaining copper oxide before evaporating, and dry the crystals for longer between fresh filter paper.
Question 13
(a) M1 15.0 x (2.0 / 5.0), or equivalent scaling/ratio method
(a) A1 6.0 (cm3) cao
(a) Answer: 6.0 cm3
(b) B1 the pH would fall below 7 (the mixture becomes acidic overall), oe
(b) B1 universal indicator would turn an acidic colour (orange/red) instead of green, oe
(b) Answer: The mixture would become acidic (pH below 7), and universal indicator would turn orange or red instead of green.
Question 14
(a) M1 magnesium first and copper last
(a) A1 magnesium, zinc, iron, copper (fully correct order) cao
(a) Answer: Magnesium, zinc, iron, copper
(b) M1 correctly links faster bubbling to greater reactivity
(b) A1 W, X, Y, Z (fully correct order) cao
(b) Answer: W, X, Y, Z
Question 15
(a) B1 iron sulfate, oe cao
(a) B1 copper cao
(a) Answer: iron sulfate + copper
(b) B1 iron is more reactive than copper, oe
(b) B1 so iron atoms displace (push out) copper from the copper sulfate solution, forming iron sulfate and releasing copper metal, oe
(b) Answer: Iron is more reactive than copper, so iron displaces copper from the copper sulfate solution, forming iron sulfate and depositing copper metal onto the nail.
(c) B1 no visible reaction would take place, oe
(c) B1 because copper is less reactive than iron, so it cannot displace iron from the iron sulfate solution, oe
(c) Answer: No reaction would occur, because copper is less reactive than iron and so cannot displace iron from iron sulfate solution.
Question 16
(a) B1 water (moisture) cao
(a) B1 oxygen (air) cao
(a) Answer: Water and oxygen
(b) B1 tube C only, cao
(b) B1 tube A has no water present (the drying agent removes moisture), so rusting cannot occur even though oxygen is present, oe
(b) B1 tube B has boiled water (removing dissolved oxygen) sealed under a layer of oil that stops air/oxygen getting in, so rusting cannot occur even though water is present, oe
(b) Answer: Only tube C rusts. Tube A lacks water (removed by the drying agent) and tube B lacks oxygen (boiled out of the water and sealed by the oil layer), so neither tube has both conditions needed.
(c) B1 any valid control variable, e.g. size/type/mass of iron nail used, oe
(c) B1 correct reasoning: so that any difference seen is only due to the water/oxygen conditions, not a difference between the nails themselves, oe
(c) Answer: Use identical iron nails (same size, type and mass) in each tube, so that any difference in rusting is caused only by the different conditions, not by differences between the nails.
Question 17
Level 1 (1-2): A basic, undeveloped answer. States that one or more methods can prevent rusting, with little or no scientific detail on how they work and no reasoned comparison or recommendation.
Level 2 (3-4): Describes how at least two of the three methods work (e.g. barrier methods and/or sacrificial protection) with some correct scientific detail. Attempts a comparison or recommendation, but reasoning is limited or only partly linked to the ship's hull scenario.
Level 3 (5-6): Accurately describes how all three methods work, correctly using ideas of barrier protection and sacrificial protection linked to reactivity. Gives a clearly justified evaluation of which method (or combination) is most suitable for a ship's hull, with reasoning based on the seawater environment and practicality of maintenance.
Indicative content:
Painting forms a barrier that stops water and oxygen reaching the iron surface, preventing rusting.
Painting is cheap and gives a choice of colour/appearance, but the coating is easily scratched or damaged, and once broken the exposed iron underneath will rust; it needs regular reapplication.
Galvanising coats the iron with a layer of zinc, which acts as a barrier but also as sacrificial protection because zinc is more reactive than iron.
Because zinc is more reactive, if the zinc coating is scratched the zinc reacts with water and oxygen instead of the iron, so the iron underneath is still protected even when the coating is damaged.
Sacrificial protection attaches blocks of a metal more reactive than iron, such as magnesium or zinc, directly onto the structure.
The more reactive metal corrodes (reacts) in place of the iron, because it loses electrons more easily/reacts more readily with water and oxygen than iron does; the blocks need to be checked and replaced periodically as they wear away.
For a ship's hull in seawater, paint alone is not ideal because the coating is likely to be damaged by waves, floating debris and general wear, quickly exposing bare iron to constant seawater.
Sacrificial protection (often combined with painting) is well suited to a ship's hull because it continues to protect the iron even where the surface coating has been scratched or worn away, and sacrificial blocks can be inspected and replaced without needing to fully repaint the hull.
A justified conclusion should recognise that using more than one method together (e.g. paint plus sacrificial anodes) gives the most reliable long-term protection for a hull that is constantly exposed to seawater.
Question 18
(a) B1 a substance that increases the rate of a reaction without being used up (chemically unchanged at the end), oe cao
(a) Answer: A substance that speeds up a chemical reaction without being used up itself.
(b) B1 the catalyst provides an alternative reaction pathway that needs less energy (lower activation energy), oe
(b) B1 so a greater proportion of particle collisions have enough energy to react successfully, increasing the frequency of successful collisions and therefore the rate of reaction, oe
(b) Answer: The catalyst provides an alternative pathway with a lower activation energy, so more particle collisions have enough energy to succeed, increasing the rate of successful collisions.
(c) B1 with a lower activation energy, a larger proportion of colliding particles already have enough energy to react, oe
(c) B1 so more of the total collisions result in a reaction (become successful collisions), even though the total number of collisions taking place has not increased, oe
(c) Answer: Because less energy is now needed for a successful collision, a larger fraction of all the collisions taking place have enough energy to react, so more collisions succeed even though the total number of collisions is unchanged.
Question 19
(a) B1 carbon dioxide and water, both needed, oe cao
(a) Answer: carbon dioxide + water
(b) B1 2 in front of O2 on the left
(b) B1 2 in front of H2O on the right, fully balanced: CH4 + 2O2 -> CO2 + 2H2O, oe cao
(b) Answer: CH4 + 2O2 -> CO2 + 2H2O
(c) M1 4.00 + 16.00 - 11.00
(c) A1 9.00 (g) cao
(c) Answer: 9.00 g
Question 20
(a) M1 increase in mass = 4.00 - 2.40 = 1.60 (g); percentage = (1.60 / 2.40) x 100
(a) A1 66.7% shown to at least 1 dp, cso
(a) Answer: 66.7% (as given)
(b) M1 40 - 24, or correct rearrangement of RFM(MgO) = RAM(Mg) + RAM(O)
(b) A1 16 cao
(b) Answer: 16
Question 21
Level 1 (1-2): A basic statement is made, e.g. that more concentrated acid reacts faster, without linking this to collision theory or explaining why the final volumes are the same. Little or no evaluation of a suitable concentration for a school laboratory.
Level 2 (3-4): Links the steepness of at least one curve to particle collision frequency, and/or correctly explains why the curves level off at the same final volume. Attempts an evaluation of a suitable concentration for school use, but reasoning is limited.
Level 3 (5-6): Clearly explains, using collision theory, why increasing acid concentration increases the initial gradient of the curves (more acid particles per unit volume, more frequent collisions with the marble chip surface). Correctly explains that the final volume is the same in each experiment because the same small mass of marble chips (the limiting reactant) is fully used up regardless of acid concentration. Gives a well-justified evaluation of the most suitable concentration for a school laboratory investigation, with valid reasoning (e.g. balancing reaction time, ease of reading the gas syringe, and safety/hazard of handling concentrated acid).
Indicative content:
Concentrated acid (curve C) has the highest concentration of acid particles per unit volume, so collisions between acid particles and the marble chip surface happen more frequently.
This higher collision frequency increases the frequency of successful collisions, so the reaction proceeds fastest at first, giving the steepest initial gradient.
Dilute acid (curve A) has fewer acid particles in the same volume, so collisions with the marble surface are less frequent, giving the shallowest initial gradient and the slowest reaction.
Medium concentration (curve B) lies between the two, as expected from its intermediate acid particle concentration.
All three curves eventually become horizontal (level off) at the same final gas volume, because the same small mass of marble chips is the limiting reactant in every experiment and is completely used up regardless of how concentrated the acid is.
Evaluation: a medium concentration would likely be most suitable for a school laboratory, since it reacts fast enough to finish within a normal lesson while still being slow enough to read the gas syringe scale accurately at regular time intervals.
Evaluation: concentrated acid may react too quickly to record volumes accurately, and is more hazardous/corrosive to handle, while dilute acid may take too long to complete within a lesson.
A valid, well-reasoned evaluation referencing at least two of time, measurability and safety should be credited at the top level.